... The ASxxxx Assemblers
Assembly Pass 1 During pass 1, ASxxxx opens all source files and performs a rudimenatry assembly of each source statement. During this pro- cess all symbol tables are built, program sections defined, and number of bytes for each assembled source line is estimated. At the end of pass 1 all undefined symbols may be made global (external) using the ASxxxx switch -g, otherwise undefined sym- bols will be flagged as errors during succeeding passes.
Assembly Pass 2 During pass 2 the ASxxxx assembler resolves forward refer- ences and determines the number of bytes for each assembled line. The number of bytes used by a particular assembler in- struction may depend upon the addressing mode, whether the in- struction allows multiple forms based upon the relative distance to the addressed location, or other factors. Pass 2 resolves these cases and determines the address of all symbols. Those assemblers with multiple forms are able to automatically repeat pass 2 as many times as necessary to resolve all differences in instruction lengths and forward references. All other assem- blers can manually specify additional passes to resolve more than one level of forward referencing.
Assembly Pass 3 Pass 3 by the assembler generates the listing file, the relo- catable output file, and the symbol tables. Also during pass 3 the errors will be reported. The relocatable object file is an ascii file containing sym- bol references and definitions, program area definitions, and the relocatable assembled code, the linker ASLINK will use this information to generate an absolute load file (Intel, Motorola or Tandy CoCo Disk Basic formats).
... Source Program Format
Statement Format
A source program is composed of assembly-language statements.
Each statement must be completed on one line. A line may con-
tain a maximum of 128 characters, longer lines are truncated and
lost.
An ASxxxx assembler statement may have as many as four
fields. These fields are identified by their order within the
statement and/or by separating characters between fields. The
general format of the ASxxxx statement is:
[label:] Operator Operand [;Comment(s)]
The label and comment fields are optional. The operator and
operand fields are interdependent. The operator field may be an
assembler directive or an assembly mnemonic. The operand field
may be optional or required as defined in the context of the
operator.
ASxxxx interprets and processes source statements one at a
time. Each statement causes a particular operation to be per-
formed.
Label Field -
A label is a user-defined symbol which is assigned the value
of the current location counter and entered into the user de-
fined symbol table. The current location counter is used by
ASxxxx to assign memory addresses to the source program state-
ments as they are encountered during the assembly process. Thus
a label is a means of symbolically referring to a specific
statement.
When a program section is absolute, the value of the current
location counter is absolute; its value references an absolute
memory address. Similarly, when a program section is relocat-
able, the value of the current location counter is relocatable.
A relocation bias calculated at link time is added to the ap-
parent value of the current location counter to establish its
effective absolute address at execution time. (The user can
also force the linker to relocate sections defined as absolute.
This may be required under special circumstances.)
If present, a label must be the first field in a source
statement and must be terminated by a colon (:). For example,
if the value of the current location counter is absolute
01F0(H), the statement:
abcd: nop
assigns the value 01F0(H) to the label abcd. If the location
counter value were relocatable, the final value of abcd would be
01F0(H)+K, where K represents the relocation bias of the program
section, as calculated by the linker at link time.
More than one label may appear within a single label field.
Each label so specified is assigned the same address value. For
example, if the value of the current location counter is
1FF0(H), the multiple labels in the following statement are each
assigned the value 1FF0(H):
abcd: aq: $abc: nop
Multiple labels may also appear on successive lines. For ex-
ample, the statements
abcd:
aq:
$abc: nop
likewise cause the same value to be assigned to all three la-
bels.
A double colon (::) defines the label as a global symbol.
For example, the statement
abcd:: nop
establishes the label abcd as a global symbol. The distinguish-
ing attribute of a global symbol is that it can be referenced
from within an object module other than the module in which the
symbol is defined. References to this label in other modules
are resolved when the modules are linked as a composite execut-
able image.
The legal characters for defining labels are:
A through Z
a through z
0 through 9
. (Period)
$ (Dollar sign)
_ (underscore)
A label may be any length, however only the first 79
characters are significant and, therefore must be unique among
all labels in the source program (not necessarily among
separately compiled modules). An error code(s) (<m> or <p>)
will be generated in the assembly listing if the first 79
characters in two or more labels are the same. The <m> code is
caused by the redeclaration of the symbol or its reference by
another statement. The <p> code is generated because the sym-
bols location is changing on each pass through the source file.
The label must not start with the characters 0-9, as this
designates a reusable symbol with special attributes described
in a later section.
Operator Field - The operator field specifies the action to be performed. It may consist of an instruction mnemonic (op code) or an assembler directive. When the operator is an instruction mnemonic, a machine in- struction is generated and the assembler evaluates the addresses of the operands which follow. When the operator is a directive ASxxxx performs certain control actions or processing operations during assembly of the source program. Leading and trailing spaces or tabs in the operator field have no significance; such characters serve only to separate the operator field from the preceding and following fields. An operator is terminated by a space, tab or end of line.
Operand Field -
When the operator is an instruction mnemonic (op code), the
operand field contains program variables that are to be
evaluated/manipulated by the operator.
Operands may be expressions or symbols, depending on the
operator. Multiple expressions used in the operand fields may
be separated by a comma. An operand should be preceded by an
operator field; if it is not, the statement will give an error
(<q> or <o>). All operands following instruction mnemonics are
treated as expressions.
The operand field is terminated by a semicolon when the field
is followed by a comment. For example, in the following
statement:
label: lda abcd,x ;Comment field
the tab between lda and abcd terminates the operator field and
defines the beginning of the operand field; a comma separates
the operands abcd and x; and a semicolon terminates the operand
field and defines the beginning of the comment field. When no
comment field follows, the operand field is terminated by the
end of the source line.
Comment Field - The comment field begins with a semicolon and extends through the end of the line. This field is optional and may contain any 7-bit ascii character except null. Comments do not affect assembly processing or program execu- tion.
... Symbols and Expressions
This section describes the generic components of the ASxxxx assemblers: the character set, the conventions observed in con- structing symbols, and the use of numbers, operators, and ex- pressions.
Character Set
The following characters are legal in ASxxxx source programs:
1. The letters A through Z. Both upper- and lower-case
letters are acceptable. The assemblers, by default,
are case sensitive, i.e. ABCD and abcd are not the
same symbols. (The assemblers can be made case insen-
sitive by using the -z command line option.)
2. The digits 0 through 9
3. The characters . (period), $ (dollar sign), and _ (un-
derscore).
4. The special characters listed in Tables 1 through 6.
Tables 1 through 6 describe the various ASxxxx label and
field terminators, assignment operators, operand separators, as-
sembly, unary, binary, and radix operators.
Table 1 Label Terminators and Assignment Operators
----------------------------------------------------------------
: Colon Label terminator.
:: Double colon Label Terminator; defines the
label as a global label.
= Equal sign Direct assignment operator.
== Double equal Direct assignment operator;
sign defines the symbol as a global
symbol.
=: Local equal Direct assignment operator; de-
fines the symbol as a local sym-
bol.
----------------------------------------------------------------
Table 2 Field Terminators and Operand Separators
----------------------------------------------------------------
Tab Item or field terminator.
Space Item or field terminator.
, Comma Operand field separator.
; Semicolon Comment field indicator.
----------------------------------------------------------------
Table 3 Assembler Operators
----------------------------------------------------------------
# Number sign Immediate expression indicator.
. Period Current location counter.
( Left parenthesis Expression delimiter.
) Right parenthesis Expression delimeter.
----------------------------------------------------------------
Table 4 Unary Operators
----------------------------------------------------------------
< Left bracket <FEDC Produces the lower byte
value of the expression.
(DC)
> Right bracket >FEDC Produces the upper byte
value of the expression.
(FE)
+ Plus sign +A Positive value of A
- Minus sign -A Produces the negative
(2's complement) of A.
~ Tilde ~A Produces the 1's comple-
ment of A.
' Single quote 'D Produces the value of
the character D.
" Double quote "AB Produces the double byte
value for AB.
\ Backslash '\n Unix style characters
\b, \f, \n, \r, \t
or '\001 or octal byte values.
----------------------------------------------------------------
Table 5 Binary Operators
----------------------------------------------------------------
<< Double 0800 << 4 Produces the 4 bit
Left bracket left-shifted value of
0800. (8000)
>> Double 0800 >> 4 Produces the 4 bit
Right bracket right-shifted value of
0800. (0080)
+ Plus sign A + B Arithmetic Addition
operator.
- Minus sign A - B Arithmetic Subtraction
operator.
* Asterisk A * B Arithmetic Multiplica-
tion operator.
/ Slash A / B Arithmetic Division
operator.
& Ampersand A & B Logical AND operator.
| Bar A | B Logical OR operator.
% Percent sign A % B Modulus operator.
^ Up arrow or A ^ B EXCLUSIVE OR operator.
circumflex
----------------------------------------------------------------
Table 6 Temporary Radix Operators
----------------------------------------------------------------
$%, ^b, ^B, 0b, 0B Binary radix operator.
$&, ^o, ^O, 0o, 0O Octal radix operator.
^q, ^Q, 0q, 0Q
$#, ^d, ^D, 0d, 0D Decimal radix operator.
$@, ^x, ^X, 0x, 0X Hexadecimal radix operator.
^h, ^H, 0h, 0H
Potential ambiguities arising from the use of 0b and 0d
as temporary radix operators may be circumvented by pre-
ceding all non-prefixed hexidecimal numbers with 00.
Leading 0's are required in any case where the first
hexidecimal digit is abcdef as the assembler will treat
the letter sequence as a label.
The decimal point, '.', following any numerical se-
quence not preceded by a temporary radix and containing
only the decimal digits 0-9 will be treated as a
decimal, radix 10, value.
When the 'C Style Numbers' option is enabled
(see .enabl csn) all temporary radixs beginning with a 0
(zero), except 0x and 0X, are disabled. Number se-
quences beginning with 0x or 0X are interpreted as hex,
all other numbers beginning with 0 are octal, and numer-
ical sequences not beginning with a 0 are decimal.
----------------------------------------------------------------
User-Defined Symbols
User-defined symbols are those symbols that are equated to a
specific value through a direct assignment statement or appear
as labels. These symbols are added to the User Symbol Table as
they are encountered during assembly.
The following rules govern the creation of user-defined symbols:
1. Symbols can be composed of alphanumeric characters,
dollar signs ($), periods (.), and underscores (_)
only.
2. The first character of a symbol must not be a number
(except in the case of local symbols).
3. The first 79 characters of a symbol must be unique.
A symbol can be written with more than 79 legal
characters, but the 80th and subsequent characters are
ignored.
4. Spaces and Tabs must not be embedded within a symbol.
Reusable Symbols
Reusable symbols are specially formatted symbols used as la-
bels within a block of coding that has been delimited as a reus-
able symbol block. Reusable symbols are of the form n$, where n
is a decimal integer from 0 to 65535, inclusive. Examples of
reusable symbols are:
1$
27$
138$
244$
The range of a reusable symbol block consists of those state-
ments between two normally constructed symbolic labels. Note
that a statement of the form:
ALPHA = EXPRESSION
is a direct assignment statement but does not create a label and
thus does not delimit the range of a reusable symbol block.
Note that the range of a reusable symbol block may extend
across program areas.
Reusable symbols provide a convenient means of generating la-
bels for branch instructions and other such references within
reusable symbol blocks. Using reusable symbols reduces the pos-
sibility of symbols with multiple definitions appearing within a
user program. In addition, the use of reusable symbols dif-
ferentiates entry-point labels from other labels, since reusable
labels cannot be referenced from outside their respective symbol
blocks. Thus, reusable symbols of the same name can appear in
other symbol blocks without conflict. Reusable symbols require
less symbol table space than normal symbols. Their use is
recommended.
The use of the same reusable symbol within a symbol block
will generate one or both of the <m> or <p> errors.
Example of reusable symbols:
a: ldx #atable ;get table address
lda #0d48 ;table length
1$: clr ,x+ ;clear
deca
bne 1$
b: ldx #btable ;get table address
lda #0d48 ;table length
1$: clr ,x+ ;clear
deca
bne 1$
Current Location Counter
The period (.) is the symbol for the current location coun-
ter. When used in the operand field of an instruction, the
period represents the address of the first byte of the
instruction:
AS: ldx #. ;The period (.) refers to
;the address of the ldx
;instruction.
When used in the operand field of an ASxxxx directive, it
represents the address of the current byte or word:
QK = 0
.word 0xFFFE,.+4,QK ;The operand .+4 in the .word
;directive represents a value
;stored in the second of the
;three words during assembly.
If we assume the current value of the program counter is
0H0200, then during assembly, ASxxxx reserves three words of
storage starting at location 0H0200. The first value, a
hexadecimal constant FFFE, will be stored at location 0H0200.
The second value represented by .+4 will be stored at location
0H0202, its value will be 0H0206 ( = 0H0202 + 4). The third
value defined by the symbol QK will be placed at location
0H0204.
At the beginning of each assembly pass, ASxxxx resets the lo-
cation counter. Normally, consecutive memory locations are as-
signed to each byte of object code generated. However, the
value of the location counter can be changed through a direct
assignment statement of the following form:
. = . + expression
The new location counter can only be specified relative to
the current location counter. Neglecting to specify the current
program counter along with the expression on the right side of
the assignment operator will generate the <.> error. (Absolute
program areas may use the .org directive to specify the absolute
location of the current program counter.)
The following coding illustrates the use of the current location
counter:
.area CODE1 (ABS) ;program area CODE1
;is ABSOLUTE
.org 0H100 ;set location to
;0H100 absolute
num1: ldx #.+0H10 ;The label num1 has
;the value 0H100.
;X is loaded with
;0H100 + 0H10
.org 0H130 ;location counter
;set to 0H130
num2: ldy #. ;The label num2 has
;the value 0H130.
;Y is loaded with
;value 0H130.
.area CODE2 (REL) ;program area CODE2
;is RELOCATABLE
. = . + 0H20 ;Set location counter
;to relocatable 0H20 of
;the program section.
num3: .word 0 ;The label num3 has
;the value
;of relocatable 0H20.
. = . + 0H40 ;will reserve 0H40
;bytes of storage as will
.blkb 0H40 ;or
.blkw 0H20
The .blkb and .blkw directives are the preferred methods of
allocating space.
Numbers ASxxxx assumes that all numbers in the source program are to be interpreted in decimal radix unless otherwise specified. The .radix directive may be used to specify the default as octal, decimal, or hexidecimal. Individual numbers can be designated as binary, octal, decimal, or hexidecimal through the temporary radix prefixes shown in table 6. Negative numbers must be preceded by a minus sign; ASxxxx translates such numbers into two's complement form. Positive numbers may (but need not) be preceded by a plus sign. Numbers are always considered to be absolute values, therefor they are never relocatable.
Terms
A term is a component of an expression and may be one of the
following:
1. A number.
2. A symbol:
1. A period (.) specified in an expression causes the
current location counter to be used.
2. A User-defined symbol.
3. An undefined symbol is assigned a value of zero and
inserted in the User-Defined symbol table as an
undefined symbol.
3. A single quote followed by a single ascii character, or
a double quote followed by two ascii characters.
4. An expression enclosed in parenthesis. Any expression
so enclosed is evaluated and reduced to a single term
before the remainder of the expression in which it ap-
pears is evaluated. Parenthesis, for example, may be
used to alter the left-to-right evaluation of expres-
sions, (as in A*B+C versus A*(B+C)), or to apply a un-
ary operator to an entire expression (as in -(A+B)).
5. A unary operator followed by a symbol or number.
Expressions
Expressions are combinations of terms joined together by
binary operators. Expressions reduce to a value. The evalua-
tion of an expression includes the determination of its attri-
butes. A resultant expression value may be one of three types
(as described later in this section): relocatable, absolute,
and external.
Expressions are evaluate with an operand hierarchy as follows:
* / % multiplication,
division, and
modulus first.
+ - addition and
subtraction second.
<< >> left shift and
right shift third.
^ exclusive or fourth.
& logical and fifth.
| logical or last
except that unary operators take precedence over binary
operators.
A missing or illegal operator terminates the expression
analysis, causing error codes <o> and/or <q> to be generated
depending upon the context of the expression itself.
At assembly time the value of an external (global) expression
is equal to the value of the absolute part of that expression.
For example, the expression external+4, where 'external' is an
external symbol, has the value of 4. This expression, however,
when evaluated at link time takes on the resolved value of the
symbol 'external', plus 4.
Expressions, when evaluated by ASxxxx, are one of three
types: relocatable, absolute, or external. The following dis-
tinctions are important:
1. An expression is relocatable if its value is fixed re-
lative to the base address of the program area in which
it appears; it will have an offset value added at link
time. Terms that contain labels defined in relocatable
program areas will have a relocatable value; simi-
larly, a period (.) in a relocatable program area,
representing the value of the current program location
counter, will also have a relocatable value.
2. An expression is absolute if its value is fixed. An
expression whose terms are numbers and ascii characters
will reduce to an absolute value. A relocatable ex-
pression or term minus a relocatable term, where both
elements being evaluated belong to the same program
area, is an absolute expression. This is because every
term in a program area has the same relocation bias.
When one term is subtracted from the other the reloca-
tion bias is zero.
3. An expression is external (or global) if it contains a
single global reference (plus or minus an absolute ex-
pression value) that is not defined within the current
program. Thus, an external expression is only par-
tially defined following assembly and must be resolved
at link time.
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Last Updated: September 2023